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Selecting biological control agents

Experimental confirmation of host range

Predicting indirect non-target effects of agents

What we don't know

Much of the recent research into non-target effects has concentrated on two-species interactions compared with the more subtle indirect interactions such as competition and apparent competition (Messing et al. 2006). In part this is because despite mounting information about community dynamics and how to study them, the inherent complexity and uncertainty of interactions within receiving communities makes rational decision-making about non-target effects difficult (van Lenteren et al. 2006). Babendreier et al. (2006) have reviewed the various international attempts at developing regulations for biological control, and detail the information requirements of the OECD on environmental risk assessment of biological control agents. Although information on competition and other indirect effects are now required by regulators like the OECD, it unclear how this will be achieved (Babendreier et al. 2006. With relatively few exceptions, studies reporting non-target effects provide information about effects at the level of the individual, such as percentage damage and percentage mortality. Messing et al. (2006) and others propose that evaluation of indirect effects should concentrate on population and community level effects rather than effects on individuals, and need to be field-based and long-term, to isolate temporal and spatial variation. Lockwood (2000) also considered that the conservation of 'species' as a basis for non-target impact assessment lacked depth. He made the case that a population is not a collection of individuals but the energy and nutrient flows that manifest through the individual. For this reason, in assessing impacts the emphasis should be on population and community processes, such as reproduction, competition, consumption, not on population size. Babendreier et al. (2005) suggested that we need to develop better a priori definitions of what a significant non-target effect is, and we should aspire to baseline studies before release. Such aspirations are difficult in the research-funding environment that we work in New Zealand, but this important suggestion should at least focus applicant's minds on potential issues.

How important are these indirect non-target impacts to ecosystem function? The number of deliberate introductions of exotic organisms for the purpose of biological control is small compared with the powerful influence of invasions caused by the accidental introduction of exotic organisms or for the deliberate introduction for other purposes (van Driesche and Hoddle 1997). In terms of relative risk, the role of modern biological control of pests and weeds in environmental change in New Zealand is therefore likely to be small. In a large survey of database information Lynch and Thomas (2000) found that only 1.5% of the 5000 projects they reviewed recorded adverse effects, but they acknowledged that this information was not well represented in the data. Of that 1.5%, only 10% appeared to have effects at the population level. For weed control projects in New Zealand the adverse effects are likely to be less frequent than this (Fowler et al. 2003).

There is debate about how prevalent significant adverse non-target effects are, but there is no debate that these sometimes occur. Louda et al. (2003) reviewed the known examples of adverse effects and provide views on how cases like these might be avoided in future. Simberloff and Stiling (1996) recommended care in the use of biological control because even if we could predict events such as extinction of populations, it is not obvious how to assign value. Beyond the fact that we cannot predict ripples in communities and ecosystems (let alone species) how much is the very existence of these entities worth? The uncertainty is made more acute by the irreversible nature of biological control, and ability of agents to spread. They argued that while biological control can be justified by the high threat of invasives in native communities this does not abrogate the responsibility of biological control practitioners to take care, a recommendation that seems entirely fair.

Hoddle (2004) stated that the published literature suggests that direct attacks on non-target organisms by introduced control agents can be minimised by selecting agents that are both host and habitat specific. Hawkins et al. (1999, cited in Hoddle 2004) suggest that the trophic relationships of successful biological control projects are less reticulate than those in food webs of natural systems. Less successful projects have more impact on food webs because the agent becomes permanently abundant. The best protection against subtle non-target effects is to maximise the host specificity and efficacy of agents (van Lenteren et al. 2006 and others).

Biological control practitioners who wish to introduce a control agent to New Zealand are required to assess the potential for indirect non-target impacts when it is clear that a useful framework is only just in existence. Babendreier et al. (2006) point out that the OECD framework now requires information on the indirect effects of prospective control agents, but how this can be achieved is unclear.

Messing et al. (2006) concluded that practitioners and regulators have a daunting task, as frameworks to guide prediction are rudimentary, and yet concrete and responsible decisions need to be made. They also stressed that it is essential that regulators do not extrapolate from observations of feeding or parasitism in lab or field to population effects. The EPA must operate within the limitations of our understanding of community dynamics in New Zealand ecosystems. In the face of this complexity, applicants will be asked to provide all available information, common sense interpretation of that information, and an appropriate risk assessment based on that interpretation. The following tools have been used in the interpretation of community interactions, and the examples may assist in future risk assessments.


Babendreier D., Bigler F. and Kuhlmann U. (2005). Methods Used to Assess Non-target Effects of Invertebrate Biological Control Agents of Arthropod Pests. BioControl 50: 821-870.

Babendreier D., Bigler F. and Kuhlmann U. (2006). Current status and constraints in the assessment of non-target effects. Pp. 1-13 In: Environmental impact of invertebrates for biological control of arthropods methods and risk assessment, F. , F. Bigler, D. Babendreier and U. Kuhlmann (Ed.) CABI Publishing, Wallingford, UK

Fowler S.V., Gourlay A.H., Hill R.L. and Withers T. (2003). Safety in New Zealand weed biocontrol: a retrospective analysis of host-specificity testing and the predictability of impacts on non-target plants. Pp. 265270 in Proceedings of the XI International Symposium on Biological Control of Weeds, Canberra, Australia, 2003, J.M. Cullen, D.T. Briese, D.J. Kriticos, W.M. Lonsdale, L. Morin and J.K. Scott (Ed.).

Hoddle M. (2004). Restoring balance using exotic species to control invasive exotic species. Conservation Biology 18: 38-49

Lockwood J.A. (2000). Nontarget effects of biological control: what are we trying to miss? Pp. 15-30 In: Nontarget effects of biological control introductions, P.A. Follett and J.J. Duan (Ed.) Kluwer Academic Publishers, Norwell, Massachusetts, USA.

Louda S.M., Pemberton R.W., Johnson M.T. and Follett P.A. (2003). Nontarget effects - the Achilles' heel of biological control? Retrospective analyses to reduce risk associated with biocontrol introductions. Annual Review of Entomology 48: 365-396.

Lynch L.D. and Thomas M.B. (2000). Nontarget effects in the biocontrol of insects with insects, nematodes and microbial agents: the evidence. Biocontrol News and Information 21: 117N-130N

Messing R.H., Roitberg B.D. and Brodeur J. (2006). Measuring and predicting indirect impacts of biological control, competition, displacement and secondary interactions. Pp. 64-77 In: Environmental impact of invertebrates for biological control of arthropods - methods and risk assessment, F. Bigler, D. Babendreier and U. Kuhlmann (Ed.) CABI Publishing, Wallingford, UK

Simberloff D. and Stiling P. (1996). Risks of species introduced for biological control. Biological Conservation 78: 185-192.

van Driesche R.G. and Hoddle M. (1997). Should arthropod parasitoids and predators be subject to host range testing when used as biological control agents? Agriculture and Human Values 14: 211-226.

van Lenteren J.C., Bale J., Bigler F., Hokkanen H.M.T. and Loomans A.J.M. (2006). Assessing risks of releasing exotic biological control agents of arthropod pests. Annual Review of Entomology 51: 609-634.

van Lenteren J.C., Cock M.J.W., Hoffmeister T.S. and Sands D.P.A. (2006). Host specificity in arthropod biological control, methods for testing and interpreting the data. Pp. 38-63. CAB Publishing, Delemont.